Method for monitoring the tire condition in vehicles

ABSTRACT

In a method for monitoring the tire condition in a vehicle, the vertical acceleration is measured at one axle of the vehicle, a reference stiffness value is determined from the vertical acceleration, and a tire pressure is assigned to the reference stiffness value. In the case of a detected tire pressure loss, the tire pressure is assigned to the tire of the wheel having the pressure loss.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for monitoring the tire condition in vehicles.

2. Description of Related Art

In order to monitor tire condition, it is known from published German patent document DE 103 03 492 that one may record the rotational wheel speeds of the driven wheels of a motor vehicle, a tire pressure loss being present in a wheel if this wheel has a deviating wheel speed over a longer time period. This is based on the knowledge that flattened tires have a higher rotational speed. For the improvement of the results and the more certain recognition of a tire pressure loss, additional information may go into the pressure loss detection, especially the drive torque.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is preventing critical driving conditions resulting from tire pressure loss, which tire condition is detected using simply executed measures.

In the method according to the present invention, for monitoring the tire condition in vehicles, the tire-tread circumference of the wheels, which is made the basis for detecting tire pressure losses, is determined as the vehicle parameter. A pressure loss is present if the change in the tire-tread circumference of a wheel exceeds a boundary value.

In order to be able to establish the absolute pressure of a tire encumbered with a pressure loss, the vertical acceleration is measured at one axle of the vehicle in accordance with the present invention, an associated tire stiffness value being ascertained from the vertical acceleration, to which a tire pressure is assigned. In response to roadway excitations, since the wheel and the axle react as a spring-mass system, and the spring constant of the tire is established by the tire stiffness, then if the vertical acceleration is known, a tire stiffness value may be ascertained by calculation. The inner tire pressure may be shown as a function of the tire stiffness, so that ultimately a tire's inner pressure value is able to be determined from the measurement of the vertical acceleration. That being the case, a reference of the tire encumbered with pressure loss to an absolute pressure value becomes possible.

In this instance, the concept of tire stiffness characterizes the spring properties and the damping properties of the tire, that is, the tire stiffness represents a measure of the deformation of the tire as a function of the force acting upon it.

According to one example embodiment, the vertical acceleration is ascertained on one side of the vehicle at one location per axle, e.g., in the lateral region of the axle at or in a wheel. In addition, the pressure loss of the wheels is determined indirectly, preferably via the determination of the current tire-tread circumference of the wheels. However, other alternatives are conceivable to achieve tire-tread circumference determination for the establishment of pressure loss. In the case of two reference wheels, upon pressure loss, tire stiffness change and tire-tread circumference change exist. For these wheels, pressure loss can then be directly detected. For these two sensorless wheels, only one tire-tread circumference determination is present. This is able to be referenced to the tire-tread circumferences of the wheels using vertical acceleration sensors. The advantage of this procedure may be seen in that, per axle, only one vertical acceleration sensor is sufficient to be able to establish the absolute pressure in a tire encumbered with a pressure loss.

At least one, but especially a plurality of reference values are expediently determined, during a preceding initialization step, in which an assigned reference tire stiffness value and a corresponding reference tire-tread circumference are assigned to a reference pressure. When there is pressure change, the changes of the tire-tread circumference and the associated change in the tire stiffness are then present as the measure for a pressure loss. As the reference pressure values, particularly a setpoint pressure, a lesser pressure, for instance, 25% below the setpoint pressure, and a minimum pressure are determined which, for safety reasons, should not be undershot. Reference tire stiffness values and reference tire-tread circumferences are assigned to each of these pressure reference values. In the determination of the current tire-tread circumferences/tire stiffnesses, reference may be made to the reference values, whereby the establishment of the setpoint state, a reduced pressure and a minimum pressure is able be carried out in absolute values.

It is basically sufficient to provide only one vertical acceleration sensor per axle. The position of this acceleration sensor is, for example, in or on a wheel in the lateral region of the axle; but positioning the vertical acceleration sensor in the middle region of the axle at a lateral distance from the wheels is also possible. Expediently, however, the position of the vertical acceleration sensors at different axles is in each case equal-sided with respect to the vehicles lateral direction, so as to be able to compensate for interference variables better.

Tire stiffness changes are advantageously calculated with the aid of frequency analysis, preferably using a Kalman filter or a Fast Fourier Transformation (FFT).

The axle-wise assignment of vertical acceleration sensors also has the advantage that different tires and different setpoint values per axle are taken into account. That being the case, the tires of one axle are independent of the tires of the other axle, and the respective absolute pressure values may be determined in an axle-specific manner. In the case of equal tires on all axles, it is fundamentally also possible to provide only one single acceleration sensor at only one of the axles for all the tires. The respective assignment to the tire encumbered with pressure loss takes place using the second measurement specific to the wheel, especially via the determination of the tire-tread circumference change of each wheel that is based on a rotational frequency or wheel speed measurement.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flowchart illustrating an example method for monitoring the tire condition in vehicles.

DETAILED DESCRIPTION OF THE INVENTION

To carry out the method according to the present invention shown in FIG. 1, it is assumed that one vertical acceleration sensor is situated per axle, adjacent to, or in a wheel. Furthermore, rotational wheel speeds ω_(ij) of all the wheels are determined.

First of all, an initialization is carried out in method step 1. To do this, one tire stiffness setpoint value c_(setpoint) is determined per acceleration sensor at pressure setpoint value P_(setpoint) of the tires of the respective axle, and at all four wheels an assigned tire-tread circumference setpoint value A_(setpoint) is determined. This determination takes place particularly via a reference measurement of the vertical acceleration per axle in intact tires which are filled to a setpoint pressure P_(setpoint). As further reference values, at a so-called reduced pressure P_(red) which, for example, is 25% below setpoint pressure P_(setpoint), associated tire stiffness value c_(red) and reduced tire-tread circumference A_(red) are determined. As the third reference value, in response to a minimum pressure P_(min) of the tire, an associated tire stiffness minimum value c_(min) and a tire-tread circumference minimum value A_(min) are established. The initialization of these values expediently takes place before the regular driving operation is begun, and may, for instance, be established under laboratory and test conditions by the manufacturer.

The regular method sequence, which is performed at the start of the trip, begins with method step 2. In this method step, associated vertical accelerations a_(z1), and a_(z2) are first measured, in a manner individual to each axle, with the aid of one vertical acceleration sensor per axle, respectively. In consecutive method step 3, tire stiffnesses c₁ and c₂ are ascertained from acceleration values a_(z1), and a_(z2), and from this ascertainment, in each case an associated tire pressure p₁ and p₂ are ascertained. The connection between vertical accelerations a_(z1), a_(z2) and tire stiffnesses c₁ and c₂, on the one hand, and between the tire stiffnesses and pressure values p₁, p₂, on the other hand, is known, and exists in each case as a functional interrelationship.

In next method step 4, the rotational frequencies or rotational wheel speeds ω_(ij) are measured, and from this the tire-tread circumferences A_(ij) of each individual wheel are determined. By comparing tire-tread circumferences A_(ij) from successive cycles, changes in tire-tread circumferences ΔA_(ij) of each wheel are able to be established. In addition, this makes known the relationship between the four tire-tread circumferences A_(ij), tire stiffnesses c₁ and c₂ and with that, tire pressure p₁ and p₂ that are calculated from the tire stiffnesses, and tire pressure p₃ and p₄ calculated from the comparison of the tire-tread circumferences.

In next method step 5, these tire-tread circumference changes ΔA_(ij) are then made the basis for a query by which it is checked whether the tire-tread circumference change exceeds an associated boundary value ΔA_(limit). This query takes place with the background that a change in the tire-tread circumference, that exceeds a certain measure, goes along with a corresponding pressure change in the respective tire. The tire-tread circumference decreases with increasing flattening of the tire.

If tire-tread circumference change ΔA_(ij) does not exceed the admissible boundary value ΔA_(limit) according to method step 5, the pressure change in the respective tire has not yet become relevant; in this case, subsequent to the “no” branching, the system returns to method step 2, and a new run through the method begins.

If the response to the query in method step 5 says that the tire-tread circumference change has exceeded the admissible measure, following the “yes” branching, the system continues to next method step 6, in which the assignment of the individual tire pressure p_(ij) to the axle-specific pressure value p₁ and p₂, calculated in method step 3, is carried out, depending on whether a tire of the front axle (pressure value p₁) or of the rear axle (pressure value p₂) is involved. In next method step 7, a warning signal may be emitted, in order to point out to the driver a pressure loss in the respective tire.

In subsequent method step 8, a query is made as to whether the current pressure p_(ij) that is individual to each wheel, and was ascertained in method step 6, has fallen below reduced pressure p_(red). Alternatively to this query, a query on the plane of the tire-tread circumferences may also be made, which is shown in method step 8′. According to this alternative query, it is checked whether tire-tread circumference A_(ij), that is individual to the wheel, is falling below a reduced tire-tread circumference A_(red).

If the query in method step 8 or 8′ reveals that the falling below of the respective boundary value has not yet taken place, then, following the “no” branching, the system branches back again to the beginning of the method at method step 2, in order to begin a new run through the entire method. If, however, the query reveals that the respective boundary value has already been undershot, then, after the “yes” branching the system continues to next method step 9, where a further warning signal is emitted.

In next method step 10 a further query is made to check whether the pressure value, individual to the wheel, of the tire encumbered with pressure loss, falls below a minimum pressure P_(min). Alternatively to this query, according to method step 10′, the query may also be made on the plane of the tire-tread circumferences, according to which it is checked whether tire-tread circumference A_(ij) individual to the wheel falls below an associated tire-tread circumference minimum value A_(min). If the respective boundary value has not yet been undershot, the “no” branching is subsequently returned again to the beginning of the method at method step 2. Otherwise the “yes” branching is subsequently continued to last method step 11, according to which a third warning signal is emitted.

All the warning signals that are generated in method steps 7, 9 and 11 may, additionally or alternatively, also be drawn upon internally to the vehicle, in a control unit or a regulating unit, for further processing for an indication to the driver, especially for influencing a driver assistance system such as ESP, ASR or ABS. 

1. A method for monitoring a condition of a tire on a wheel of a vehicle on a road, comprising: measuring a vehicle characteristic quantity; and detecting a tire pressure loss if one of the vehicle characteristic quantity or a quantity determined from the vehicle characteristic quantity exceeds a predetermined reference value; wherein a vertical acceleration is measured at one axle of the vehicle, and wherein a tire stiffness value is determined from the vertical acceleration, and wherein a tire pressure is assigned to the tire stiffness value, and wherein in the case of a detected pressure loss for the tire, the tire pressure is assigned to the tire having the pressure loss.
 2. The method as recited in claim 1, further comprising: measuring a tire-tread circumference of the wheel; determining a change in the tire-tread circumference of the wheel; and if the change in the tire-tread circumference of the wheel exceeds a predetermined limit value, assigning the tire pressure to the tire of the wheel.
 3. The method as recited in claim 2, wherein a reference tire stiffness value is determined based on a measurement of the vertical acceleration at at least one reference pressure, and wherein a reference tire-tread circumference is assigned to the reference tire stiffness value.
 4. The method as recited in claim 3, wherein a setpoint pressure of the tire is established as a reference pressure, and wherein a tire stiffness setpoint value and a tire-tread circumference setpoint value are determined for the setpoint pressure of the tire.
 5. The method as recited in claim 3, wherein a minimum pressure of the tire is established as a reference pressure, and wherein a tire stiffness minimum value and a tire-tread circumference minimum value are determined for the minimum pressure of the tire.
 6. The method as recited in claim 3, wherein a reduced pressure of the tire between a setpoint pressure and a minimum pressure is established as a reference pressure, and wherein a reduced tire stiffness value and a reduced tire-tread circumference are determined for the reduced pressure of the tire.
 7. The method as recited in claim 3, wherein only one vertical acceleration is measured per axle of the vehicle.
 8. The method as recited in claim 3, wherein the vertical acceleration is measured one of in or on one wheel of the axle.
 9. The method as recited in claim 3, wherein the vertical acceleration is measured at each axle of the wheel.
 10. The method as recited in claim 9, wherein the vertical acceleration is measured at each axle of the wheel on the same side of the axles.
 11. The method as recited in claim 9, wherein disturbance variables attributable to unevennesses of the road are ascertained from a displacement in time of the vertical accelerations measured at different axles.
 12. The method as recited in claim 3, wherein changes in tire stiffness are calculated by a frequency analysis using one of a Kalman filter or a Fast Fourier Transformation.
 13. A control unit for monitoring a condition of a tire on a wheel of a vehicle on a road, comprising: a means for measuring a vehicle characteristic quantity; and a means for detecting a tire pressure loss if one of the vehicle characteristic quantity or a quantity determined from the vehicle characteristic quantity exceeds a predetermined reference value; wherein a vertical acceleration is measured at one axle of the vehicle, and wherein a tire stiffness value is determined from the vertical acceleration, and wherein a tire pressure is assigned to the tire stiffness value, and wherein in the case of a detected pressure loss for the tire, the tire pressure is assigned to the tire having the pressure loss. 